ok first off a 2.5g wont cycle for you. the min for cycling would be 5g. you'll have to upgrade OR leave your 2.5g uncycled... water changes for a 2.5g would be 100% twice per week
i used small heater pads in my 2.6g and they work quite well. they arent expensive and defo wont overheat your tank caus they arent very strong... if you can find an adjustable glass heater it would be even better as they keep the temp much more stable.
if you do upgrade, obviously a heater to match the 5g is needed.
plant wont influence temp at all
keep us updated on what your gonna do