The issue is that you will probably stuck with no payout for a week.
In a meanwhile they will simply pump-dump with your money.
What are you talking about? No payout for a week from where? Who will pump dump with your money?

You need to clarify what you are talking about.
Any new pool in general has no liquid assets. So its common practice for new Pool to disable payouts for first day-two.
In our situation we are talking about the Multipool having no 365c at all (not now not in perspective), so it needs to buy it.
In place of Multipool operator any person seeing the price is risen will play with Scrypt Alt (you mined) and wait for 365 price to stabilise.
Most probably they will try also to pump-dump on 365c to catch an exploding train.
You realise a few things though right?
1) Hashcows is not a new pool, its one of the most established, popular and successful scrypt multipools out there.
2) They only need to buy enough 365Coin to cover the BTC that would have otherwise been paid out to the users who have requested 365Coin payouts instead. This isn't going to be a vast sum right now, there are only a few people taking 365Coin instead of BTC. There will be no problems buying this 365Coin with current levels of market liquidity.
3) Scrypt alts you mined are traded according to Hashcows trading strategy, this doesn't change whether you take BTC, 365, BC or any other coin for that matter. The only time Hashcows trading strategy is irrelevant is when you are keeping the alts for yourself, which is one of the unique features of this particular multipool. Their trading strategy is designed to maximise your ultimate final payout, whether thats BTC or anything else.
4) They have no reason for interest in pumping or dumping 365Coin. They just pay out in the currency you have chosen.
I think you're looking for conspiracy or problems where there are none.