If it weren't a oneshot, I'd like to see the summons come more naturally. I understand bringing them all at once for this, but I'd like to get to know them one on one in this hypothetical. The concept of a hard isekai is pretty novel considering everyone gets cheat skills. I'd limit it to keeping one permanent summon and then a temporary summon for a party of 3. The harpy girl can help get him out of danger and do some fighting while the temp summon deals with the enemy only. Or they could take turns caring for the MC, or even taking turns being the main one. 10 seems a bit excessive in that case, one or two might get forgotten about, which happens a lot in harem anime and manga.