I would prefer the larger group, but would play either way. Being the only rogue, at least my party will be able to unlock things quietly.
Seriously, though, there are ways to make the adventure a bit more difficult- some of which you have already implemented in the house rules. Not being able to spend hit dice during a short rest if we run out of food or water, for example, could be devestating. Or it may never come into play. Wands that don't recharge and potion miscibility are other ways it could become more difficult, or never have an effect. You can give enemies max hp instead of the average they have listed, and/or add enemies.
Since half the party requires a light source, we will be very unlikely to surprise anyone, removing the Surprise Attack feature from my Assassin. That will also make it very unlikely that the Gloom Stalkers will be able to remain in the dark and therefore invisible to anyone relying on Darkvision to see them. And no Cleric means no Spare the Dying, so any attempts to stabilize unconscious characters will require an action and physical contact- except for the Warlock's Healing Light, which is a bonus action and ranged.
We played with 7 characters (including two clerics), most (if not all) of whom already had magic items, and since it was a charity event, we had rerolls we could buy (and buy we did!) and everyone had a certificate for a free resurrection, two of which got used at the table, plus I had some auto-crits and Potions of Healing I earned through Extra Life donations. I don't think this will be nearly as easy as you fear it will be, but if it is we can try splitting up later on.
I rolled just above average, but a good con helps out. The bigger concern is that the fighters/rangers mostly rolled low. 
