I am reckoning that I only need 5 of these monsters at the most with proper runes on, as I will never need more than that (apart from all-nature wars) so I am thinking of adding some gold runes to increase productivity.

I have these Nature monsters at level 100:

Oikawa
Living Forest (only 1 rune slot)
Alpha Cliviast (only 1 rune slot)
Shork
El Dino Volador
Kiridar
Ape-X
Psilotus
Sarah
Yilitre
Petro Loa
Fightreer

I also have Drekk, Nemestrinus, Roastie, and Lagerchaun who are only level 50 and are used as gold slaves already.

i like very much oikawa, el dino volador, psilotus and yilitre

-psilotus for possession skills

• el dino as denier (you could use it in 3 different books)

• oikawa good damage dealer and support

• yilitre good trait and support

@Jimmy Yes, El Dino and Psilotus are my main deniers for Nature (AOE Stun/AOE possession respectively). Yilitre is a good joker to have, especially with an AOE blind.

I need those 3 monsters to fight definitely. No gold runes for them ;)

Shork and Kiridar are also my main attackers.

@Clackers

i like oikawa because he is a tank (taunt), but also a damage dealer (double damage), but also a denier (actually he has an aoe stun move that, despite the description, is 100%)

You can check gold production at wiki
there are bature with 305 . if not use 244 if not 155 then

@Haka-Taka I meant that which monsters should I not need for fighting, in which case I take way their runes and replace them with gold runes.

I just took my runes off Fightreer, Alpha Cliviast, and Living Forest, and put level 3 team gold on them.

I usually throw the gold runes on the high earners that I do not use on teams.

You already mentioned the best amongst them for wars & pvp. Psilotus, Shork, El Dino, Kiridar. I'll throw Petro in that mix as well for his versatility.
I personally like Oikawa & Groot and hope they get more credit & attention. I use em both for Dungeons and sometimes as B teams in War but that's just me.
The rest can be gold miners.