Bathing is fine so long as you keep the pigs in the warm until they are totally dry. Make sure the bathwater is warm, wrap them in a towel if any shampoos need to soak in for a time, and ideally dry the pig with a hairdryer at the end. The risk is when you take a damp pig to a place where the temperature is noticeably lower than where you bathed them, or exposing them to draughts; that's when they risk catching a chill.
Some people don't bath their pigs at all. Some bathe them every 4-6 weeks. Personally, I've noticed a big difference in the skin and coat health of my pigs if I bath them every 2 months, although I do bath the long-haired pigs pretty much every month.
The fungal shampoos are unlikely to deal with lice. I highly recommend visiting Gorgeous Guineas:
www.gorgeousguineas.com
Not only do they sell a fantastic range of general bathing shampoos (personal favourites here are Just for Boars, Posh n Go and EasiRinse), but they sell some therapeutic shampoos to treat fungal and parasitic problems. Lice 'n' Easy is the shampoo you will want for dealing with lice; you get instructions with each shampoo, but the basic idea is you wash the pig, rinse, then wash again and leave it to soak in for five minutes before rinsing and drying. You would then repeat the bath a week later, and again a week later if the lice are being extra-stubborn!
Best tips I can give in general for ridding lice, along with using the Lice n Easy shampoo, is using a nit comb and conditioner. GG sell conditioner, and the nit combs you can buy very cheap, but I found they help a lot!