I'll give your questions a shot!
To answer your first question, yes, Seekers do travel in groups. Yeomen (warrior-caste bondsmen taken from the Clan as charges) are quite a common sight, so I wouldn't be surprised to see 2+ subordinate 'Mechs under the command of the leading Warrior - especially if they plan on raiding another Clan for whatever they're looking for. It's one thing to have few charges, it's another to attack a Cluster by yourself. I think using a Binary/Nova as a cap sounds alright, with only the most important Seeker raids needing up to a Trinary. The Scorpions are known for small-unit operations, anything larger seems off.
As for the second question, it's going to depend a bit on where your unit is coming from. I've never seen anything saying that Seekers have to come from certain Galaxies, so you have the freedom to create a force using pretty much any type of unit you want. A good place to start, though, is with Chi Galaxy; it first appeared around the time ProtoMechs become available and was tailor-made for seekers. They have heavy ProtoMech and Aerospace elements to support the few 'Mechs that make up their core. If you want something to mess around with expectations, you can take a look at the Specialist Trinaries found in Beta Galaxy. I'm not sure if this is possible anymore, but they were based around using Kirghiz Cs to paradrop commando Elemental units to wreak havoc - these are the guys who studied the SLDF special forces and trained the 7th Kommando, so they're going to be as unorthodox and deadly as Clanners get.
I would not expect to see tanks, infantry, or even many BA units. Wasteful bonding of unnecessary charges is considered very dishonorable, and adding on personnel-heavy units such as infantry increases the number of support crew and supplies needed. Why take a bunch of infantry when a single person in a 'Mech can accomplish the same thing? I kinda of think of it like a pirate captain and his crew: fewer people, less gold I have to share.