That's true, i forgot about possible support slot additions. Though they did just get ATRTs and Staps.

I don't think either will see a 3rd core option for a while because of the impending specialist packs, heavies for both factions are still relatively new, same for their special forces. Repbulic just got a new commander with Anakin and Separatist got a 2nd operative and special forces with Maul and his probe droids. I'm thinking Republic gets another operative or special forces. For operative I'd say maybe Qui Gon Jinn. For special forces either the bad batch or republic commandos. For Separatists, I'm saying they either get a new commander, or a special forces. Maybe Admiral Trench for commander and Magnaguard for special forces. As far as each faction's next heavy, Republic might get an LAAT Gunship (though it is quite big), Separatist could get the Hailfire like you said.

As a fellow clone player myself, I'll try to answer your questions the best I can. Yes it's ok to have less than 10 activations. Most of the time this will be the case in fact, especially if you plan on bringing the Sabre Tank, Barcs, or ATRTs. I personally like running my ARCS as full squads when I run any. They're really good if you can get them just outside of range 3 at the end of a turn if you're running Rex. Then you can drop Rex's 2 pip, "Take that clankers" and then move them into range 3, giving them an aim because of tactical 1, and unloading a powerful 1 black 1 white die per mini attack (plus the heavy if you run them with one). However, the strike teams are very good too, getting that activation count up. Note: you must run them with a heavy as per the rule on the strike team's card. While Phase IIs are pretty good (mmm I love me those surges from reliable), their heavy weapon options are lacking, with the z6 being technically no better than the Phase 1s version, and the mortar being subpar due to cumbersome and not being very effective against Separatists. Phase 1s on the other hand have a number of decent heavy weapon options depending on your playstyle as well as a useful trooper upgrade (Phase 1 clone specialists). But yes, I'd say Fives will generally be best in a Phase 1 squad regardless.

B2s are back up as well!