This has been a topic I've been reevaluating and researching lately so here is my shot at it. I use to think that all the successor states basically used something akin to RCTs but the more I read into it the less it looked that way. In fact it looks like the different states have very different strategies when it comes to deciding what they bring to the front.
The Davions have the most combined-arms heavy doctrine by a large margin as their RCT armor brigades and later LCTs are unique in the IS. It takes a lot of transportation assets to move an RCT but Davion makes up for this by placing large number of mechs in March Militias that, according to the Davion field manual, have extremely limited organic transport and are reliant on a small pool of Jumpships and Dropships controlled by the march lord to move as opposed to the front line formations that have organic Dropship and occasionally Jumpship support. How fleet assets are assigned is a big deal considering Dropships cost more than twice the price of the units they carry.
The transport bottleneck means that in practice, in situations where Davion can't bring superior force to bear, the RCTs tend to get ganged up on by pairs of enemy regiments like in FedSuns/Combine skirmish during the Fedcom Civil War. RCTs are usually big and bad enough that they can handle this but it does mean that Davion is frequently outnumbered in mechs. In these situations Davion tends to prefer the flexibility that comes from having Hovercraft, VTOLs, and artillery than larger numbers of mechs. It should also be noted that Davion tends to supplement its front line RCTs with mercenaries and independent mech regiments bringing the ratio of mechs to armor down to a little under 2 or 1.5 tanks to 1 mech in practice rather than the 3 to 1 ratio seen in large RCTs.
Steiner took up the Davion system in the FedCom days but had proportionally fewer of its line units as RCTs. From the Lyran Alliance field manuals it seemed that Steiner was cutting corners with upgraded and padding out the conventional and aerospace assets of its RCTs in favor of upgrading its mechs. Before Steiner took up RCTs it would combine its mech regiments with at least three regiments of supporting conventional units, which is not too different than the way the other successor states do it.
Marik, Liao, Kurita, and most of the periphery and minor powers take a more mech centric approach and for the most part keep their conventional assets assigned to planetary garrisons. This isn't to say they have less armored regiments overall or are even less skilled than Davion's, the CapCon field manual specifically says their armor crews are better trained than most others, but that they tend to assign their scarce fleet transportation assets to their mechs forces over their conventional units. On the offensive these houses usually only attach a battalion or regiment of armor and regiment or two of infantry to their mech forces and only occasionally attach independent armor regiments to their invasion forces. The Combine field manual specifically says they that it is unusual for the Combine to detach armor regiments for offensive operations and the fluff material backs this up. From the fluff it looks like both the FWLM and the CCAF also only occasionally attach independent armor regiments to their offensive operations and prefer to attack with mostly mech assets.
Of all these houses the Combine stands out in its field manual as being the least combined arms with regards to infantry and armor and the manual even states them as being sluggish with regards to the adoption of battle armor. However, the Combine does make great use of air power and proportionally has the highest ratio of Aerospace fighters to ground forces of the five houses. Comparatively Marik tends to have more infantry as their infantry regiments are double strength. Their very formidable and prolific battle armor assets tend to back this up. The Cappellans don't stand out much with regards to conventional but, if you read the unit fluffs in their field manual, they do tend to pad out their conventional infantry and armor assets with lots of artillery.
The clans have their own system of combined arms with the "typical" cluster having 3 mech trinaries, 1 elemental trinary, and 1 aerospace trinary. I say "typical" because many clans vary these schema a little one way or another. If you add a command star of mechs and make one of the trinaries a supernova trinary you get 50 mechs, 30 elemental points, and 30 aerospace fighters. This means the clans have an extremely high ratio of airpower to land forces. Even those clusters that skimp on airpower and only have a binary of 20 fighters still have a higher ratio than any successor state. This doesn't seem to come up much in the novels but a single clan cluster has, with the tech difference, more airpower than a Davion RCT.
The Comguards don't seem to have any independent armor regiments but they do have some armor in their divisions. A Comguard division on average tends to be a regiment mechs, a couple companies of armor, a regiment of infantry, and two wings of ASF. Proportionally this is pretty similar overall to what the Combine brings with their mech assets but the guards spice things up by mixing their assets down the small unit level. I should also add that the Cappellans also like to mix armor and mechs on a small scale with their demi-lances and demi-companies that are mixed armor and mech formations.
