There are no restrictions on this, so theoretically one could use a Tinkhec on a Tinkhecitd make for a pretty huge monster, considering Tinkhec starts out at 2700 ATK
It may feel clunky at first, but after playing a few rounds, the game starts to feel more smooth and has a flow to it
Ankle Shanker Bane of the Living Beacon of Unrest Blood Tyrant Consuming Aberration Corpsejack Menace Crackling Doom Cruel Entertainment Curse of Vengeance Curtains' Call Duneblast Everlasting Torment Glint-Eye Nephilim Guiltfeeder In Garruk's Wake Languish Lavalanche Magus of the Will Merciless Eviction Nath of the Gilt-Leaf Necroplasm Reyhan, Last of the Abzan Sangromancer Sphinx Summoner Tymna the Weaver Utter End Vulturous Zombie Waste Not Whispering Madness Worm Harvest
C Common Rainbow Refraction X SPELL Quick-Play If a monster(s) you control whose original name is "Rainbow Dragon" or "Rainbow Dark Dragon" activated its effect this turn: Special Summon any number of "Crystal Beast" monsters with different names from your Deck
A few standout choices include Kenrith, the Returned King, Agrus Kos, Eternal Soldier, Tazri, Stalwart Survivor, and Dynaheir, Invoker Adept
Note that it gives more than just creatures hexproof, and getting hexproof yourself as a bonus isnt something to ignore