How about mode where....

1) Instead of regular Borilla wave replaced with extra hard mode borilla
2) instead of 2 Borillas - 2 rhinos
3) Instead of Boarior - Actually... I don't have suggestions here
4) Instead of 2 rhinos - 2 hard mode Boariors (but instead of extra hard mode crocs - they spawn fire crocs) and they both have shared health
5) Instead of Swineclops - it's hard mode Swineclops with 2 extra hard mode Borillas that have follower AI (similiar to Crocs-pigs) and Borillas + Swineclops have shared health bar

But my main suggestion is for #4 and #5, you can call mode what ever you want :)

the replacing mobs of the original wave mobs seem to be have bigger attack range then what they used to back in original Random swap mode. Is size ratio corect when forge mob/boss replacing original mob from the wave? Their attack range is bigger. You used to able to melee tiny swineclops when they replace pigs while have small window of free hits before tiny swineclops start to punch you

That is a side effect of switching to Reforged's version of scaling. I will consider if it needs to be adjusted for fidelity.

Sorry for the delayed response.

With respect to wavesets, I prefer for mob interactions to not stray too far from the original unless you have explicitly picked those settings. If you have Extra Hard behaviour in Normal difficulty what is it going to do in Hard and Extra Hard?

I have added a link to a ReForged Mod Template on this mod page if you wish to implement something as you have described. It should be simple to do as it'd mostly be combining some things that I have already done.