Based on testing via tabletop, all they'd have to do is give monsters proper stats and a 6 party size would work just fine.

Example: start game with 4 instead of 1. Meet Lae'zel. Party of 5. Fight 3 imps with proper 5e stats - sting, invisibility and resistance - balanced first encounter. Not too hard for 5. Good first battle to get you used to the game. Meet Shadowheart. Party of 6. Fight 3 and then 3 more enemies, 2 imps + hellsboar each fight... Balanced for 6 with proper stats. Not too hard but certainly not easy for level 1.

On beach. 3 intellect devourers with proper stats, party of 5 with Shadowheart. Tough fight, but doable as long as you follow SH advice and keep your distance.

So they wouldn't need more enemies. Just proper 5e stats for a party of 6 max.