You can only play with the heroes you have. While it would be better to put fast heroes on the wing, you can’t do that if you don’t have any.
There are some posts about the ideal placement of heroes that you may find useful.
You’ve already received some good advice. There is another rule of thumb that you should put weak colors next to strong so that as an opponent stacks blue against your red tank, some of those blue tiles will “splash” onto your blue hero and thereby charge her/him. It’s not a hard-and-fast rule, but a consideration.
I would reconsider pairing Wilbur with Isarnia since they both have a defense down special. Wilbur’s is higher, but as @Sarah2 points out, if you stack red on defense, you’re making it easy on your attackers. IMO, rainbow is the way to go – there are both exceptions based on your roster (for a while I had Hel and Seshat flanking my Guinevere tank but as soon as I got Telluria, I went right back to rainbow) and some folks who philosophically disagree.
Who do you have for blue, yellow, and green heroes?
Btw, of the three heroes your mention, Quintus, Elena, and Isarnia, I think Isarnia makes the best tank. Quintus deals damage, but Isarnia’s defense down will make subsequent tiles more damaging. Just my opinion.