My current Squigly trio is BB/Squigs/xx Practicing Double for the xx position, but a bunch of characters work there.
My team theory:
I'm running BB on point because SSJ is big enough that Opera is actually a safe DHC when you cancel into it properly and because I like having BB with Squigly 2HP assist, for an almost GG Eddie like reset that I like to run.
ie. assist + j.MK IOH, not complicated at all but FAST and you can easily just bait reversals instead, it is a 50/50 which has to be blocked first so its a very reliable reset, one of my favorites in the game, you can run it 3/4 times in a row on one person and it is still incredibly effective.
Squigly and Big Band have a fair bit of synergy in general, the other way around, Squigly has a consistent reversal, though using 2 meters, in Daisy Pusher xx SSJ, expensive but Squigs doesn't really have good options on her own in that situation.
HP knuckle assist is fantastic for Squigly, helps as a GTFO as well as allowing a loooong time to charge stances whenever you want them.
Oh, You can also MK+HK Opera out of the last hit of SSJ and continue the combo, that's another little thing that's nice.
For last position, there are a few good choices I think:
Double is obvious and I'm practicing her to fit on both my main teams, because, she's just plain that good as a support character, she's ridiculous. Safe DHC into pressure whenever you want, soild anchor play, especially with meter stocked and Bomber is just a really, really useful assist, obviously.
Cerebella is also pretty great, Cerecopter is one of Squigly's best assists, but only on a team of 3 I believe, if you like to run Bella last I'd almost consider running Squigs as a point with a reversal assist in second, (say... Double?) especially one who also will provide a safe DHC. (say... Double?)
The other choices are mostly the other standard fare, I feel Fillia is a slightly less synergistic choice, but she works because she's Fillia. Fortune could work, again because she's just an amazing character in her own right. Squigly will struggle to convert from fiber upper though... so rememebr that.
Honourable mention to Parasoul, who I feel runs slightly better than the 2 I just mentioned, though not as well as the first 2. I've ran her quite a lot on the team and think she can be pretty effective, offering a safe DHC in some situations, just be aware that she is probably the weakest anchor of everyone mentioned here.