I usually keep them separate. Within an infantry unit (battalion, regiment etc.) you may have conventional infantry and battle armor infantry companies. I have seen this referenced in canon in a few places. Like in the FWLM back when BA was relatively new in the 3050s-3060s they would add a BA platoon or company to a conventional infantry regiment.
Most of those early days (when BA was new and scarce) references that spoke of an infantry unit having a squad or platoon of BA, made them sound like those assets were at the disposal of senior commanders. That gave me more of the vibe that they were organized separately. Perhaps attached to a headquarters. Rather then melded into a conventional infantry company.
We have seen some canon mixed infantry battalions and regiments.
But I'm not sure I'd mix them at the company level. At least on paper, on the organizational chart. I've sketched out a few infantry regiments over the years that had a company of BA in there somewhere, sometime within one of the battalions, sometimes as a company attached to the regimental headquarters.
I agree with AlphaMirage that jump infantry is a likely candidate for mixing. That gives the jump infantry unit a good boost of armor/firepower in the form of BA support. A lot of jump infantry have few to no support weapons or heavy weapons, so that adds something meaningful.
Another area/category I could see is a battle armor unit (primarily) that has a few conventional infantry elements. A security company, or a scout company (perhaps motorized infantry) that kind of thing, perhaps attached to a BA battalion headquarters. Most battle armor designs also realistically would struggle with things like clearing buildings and so bringing some conventional infantry into the mix for that is something I could see. Whether that's assembling an ad hoc combat command or something on a more permanent basis. I still think in terms of permanent organization I'd keep them separate at the company level for permanent organization/parent unit purposes, but I can see them being tasked to work together pretty often.
They are just very different. There are fewer viable dropship options that can carry both at the same time. They have different logistical needs and operational parameters. Battle Armor can expect to operate for a matter of hours or a day before they need to recharge their suits and they need different specialized repair equipment and technicians. Conventional infantry have different factors impacting their combat endurance and different support needs. So you have elements within the same company with vastly different operational parameters and logistical needs. That sounds tough to manage to me.
I think about that and to me it represents the kind of challenge you need a staff for, and it's mostly at the battalion level and up that you start to see the more specialized staff headquarters, including staff officers, staff contingents and deeper levels of support services for a combat unit, with a HQ and support elements that are going to be a bit farther behind the front lines (company HQ elements on down are usually very close to the front). So, for me it's at that level that mixing and matching different types of assets starts to look more realistic and feasible.
I apply this logic to almost anything. A combined-arms battalion (or regiment) of any mix becomes a lot more plausible to me because there's a more well-rounded headquarters sitting on top of it. A combined-arms company is challenging. (I don't mean ad hoc combat commands or battle taskforces assembled for a specific mission or task, I mean paper organization)
Are there, have there been, units that do it? Sure. Com Guards, WoB, Ceti Hussars, Eridani Light Horse, some mercs etc. But in my eyes, it's just not how most units and most militaries do it. That was part of the intrigue surrounding those militaries and those units, they were different.
