Well, I wouldn't recommend closing your main needle all the way in and then opening it 1/4 of a turn at a time till your motor runs - you could end up with the 2.5 firing up on a really unhealthy ultra lean setting and wasting an engine as a consequence.

Starting from 3 3/4 turns out, like litsluv suggested, sounds more like it. Then gradually lean the mixture 1/8th of a turn at a time til you find that sweet spot - on the 2.5, it's very narrow and sensitive to adjustments.

Fuel can absorb water, but it depends how you store it. If you keep it outside and you live in an area with high humidity, then the fuel won't keep very well. I keep my fuel indoors. Metal fuel containers begin to corrode after a while, so plastic containers are probably better if you don't use your fuel quickly.

However, I wouldn't pour away 1 month old fuel based on my LHS's advice!! Your's has probably just started seeing $$ signs in his eyes!!

You need to get the right glow plug, depending on what fuel you intend to run. If you're running low nitro fuel, a medium plug will probably be OK. With higher nitro content fuel, you'll need colder plugs. Having the wrong plug can lead to inconsistent idle and premature plug failure.