This has been a rather interesting question I haven't really dug too deep into, if not for my own current development projects, at least then for other games I've played, where formations are a thing.
I do know that in the case of Suikoden II, formation was generally simple, as in, 2 rows of 3 positions each, for a maximum of 6 party characters (plus, 2 extra characters for special events, but that doesn't count for formation). You can put short-ranged/weapon'd fighters in front, long-range attackers and healers in back, but there were also medium-ranged characters that could go either front or back. The short-ranged ones would not be able to do anything in their turn, if they ended up in a back-row position. So, for that, it was straightforward -- except in my case, I didn't really think of them in terms of "tank", "healer", or whatever, I only look for an 'S', 'M', or 'L' as the need may be.
Not having actually finished an RM game yet, or even come to the point of battle systems, I guess that formation is entirely dependent upon the situation and/or game. For example, in the typical "medieval fantasy" that RM is based in, formation would be almost intensely important. But if I were to create an RM scifi-themed game where combat positions were not attacks from the characters, but rather the result of characters manning key ship systems, then formation isn't so important to me, because I'd probably jury-rig a battle layout showing both opponents and party characters at their ship stations -- or, would my "battle layout" be equivalent of a "formation"?
It's questions like that, which would keep me awake all night. (As if I didn't already have enough to think about.)