Learn how to read, because you clearly still don't understand what these 2 told you yet. Basically what they said:

1. Fully leveled, fully fed Bunta is stronger in battles and gives more power to entire group than fully leveled, fully fed Serpent.

2. Yes, feeding all 3 summon is near impossible. They are telling you to let Serpent starve and use the chakras you'd normally use on Serpent to lv up Bunta instead.

3. Yes, in the short term future, you group BP/gnw will suffer since Serpent will be starving and Bunta is still leveling. But in the long term future, it'd be better than your current situation.

In the end, ask yourself these: How long do you plan to play? How long do you plan to stay separate as an inactive group? Because in my experience, inactive group like yours usually just end up merging into a more active group. So if you see that as an eventual future, do whatever with summon. It won't matter anyway. But if you plan to stay for a long time and don't have/see any such plan, do as they advice and get Bunta, start leveling him, and just abandon Serpent. By abandon, I mean, stop feeding him, you can still do summon capture to farm cps on him.