need a lot bigger tank 75g+ is min for Lepomis species. Enneacanthus could work but there not from there. bluegill get big even by Lepomis standards and are very aggressive. some darters or Topminnows would work from your region. always check laws about collecting 1st and be 100% sure to be able to identify what you have.
Yep the 36g is too small in the long run for sure and the 75g is too small for larger species (Bluegill/Redear Sunfish/Green Sunfish) as well if you are keeping a group. Bluegill can get very large actually over 12" and up to about 2lbs if they are normal strain Bluegill, the Georgia Giant strain can hit 5lbs and 16" or so. I've attached a pic of a beast normal strain one for reference.
Agreed with above. Sunfish do best either alone or in groups of 5-6+ to spread out aggression. If you want a group, then a 75g would be perfect for half a dozen as long as you keep up with water changes. My female Green (fully* grown 5.5") does very well alone in a 30 long but I wouldn't put anything else with her.
Agree with keeping multiple or a single one for aggression as well. I've kept plenty of sunfish species through the years and even in a large tank for them (240g) they picked on one another pretty bad when I had smaller groups of them. I've kept Bluegill, Green Sunfish, Orange Spotted, and Redear thus far. My Green even started creating nesting sites despite being the only one in the tank and claimed an area where it was very aggressive towards any other tank mates even those twice its size. The green can get a lot bigger than 5.5" I've personally caught them to close to 10" from a friends stock pond and about 3/4lb.
I'd recommend a smaller sunfish species like Lepomis humilis the Orange Spotted Sunfish for several reasons instead of Bluegill. For starters they stay much smaller with max size being under 6" and they are also far more colorful and look better in the aquarium as well as being more peaceful than Bluegill. You can find them for sale online fairly easily or on Aquabid.