so I was wondering, if somebody has flick in their town, can I visit and get a commission from him? and if so, can I visit him in multiple towns on the same day and get multiple commissions?

I want to get a bunch of butterfly models to put around a fountain by my museum, but considering how uncommon he is, that's gonna take a while. seven weeks minimum if I somehow get lucky and have him every week

No you can't sell or get commissions from Flick in any island but your own.

I can't say for sure but I read on here somewhere that you can't sell to him in other towns, so I would imagine you can't get commissions.
If you have multiple characters though each one can get their own commission so that could be 8 a visit.  I did that to expedite things a bit.

ah well, that's a bummer. maybe I'll have to make some temp characters next time he comes if I can find room for all their tents

You should get him at a minimum once every 3 weeks, as he is an optional visitor. Save high value bugs until he shows up. I had him last week and sold him a load of scorpions and other bugs for just over 700,000 bells.