good question!
well yes it can be ok to keep Crays in a community tank, and no it isn't safe to do it.
First off you need to make sure the Crays you are being offered are Australian Redclaw
(Cherax quadricarinatus) also called the Blue lobster, that is the only legal Cray you can keep in the UK.
Blue Crays are not a breed, they are a genetic defect! so you will get Blue Crays of all species, so you will need to check. before buying as stated above. if the lfs cant sex the Cray, or at least tell you how to, you may think of buying somewhere else.
The age of the community is important, young community's are not right. same goes for the Cray, a very young Cray will do badly in a community. the personality of both is important too, a tank full of nippers, will lead to the Cray taking revenge at a later date. but i can see no problems with your stock, with the exception of the Tiger Barbs, if the are not a problem currently, then they are unlikely to become so. you will need to provide a few hides for your Cray to use. lol plastic drain pipe will do. they need well oxygenated water, but apart from that they are very tolerant. indeed if you keep one on its own, you will not need to do water changes, just topping up after evaporation. The first few days will give you all the information you need to decide if it is going to work. they are scavengers, not hunters, so if you keep it well fed, again i see no problems.
lol i dont subscribe to the, "No pincers with fins" point of view, a point of view, usually but not always, help by people who have read it. I keep Redcrab, been in the tank six months, and a female Cray, been in over two months, and have yet to see a nipped fin!! lol both do their best to avoid fish. though the Cray will move fish, on if they hang around too much.
they eat anything, meat, fish flake, bloodworm, cucumber. fresh peas, but mine loves fresh cooked chicken. they cast about every three weeks, when younger , about once a year as they age. lifespan is 3-5 years, they grow to between 5-8 inches, nose to tail, add pincers they can reach 10 inches. ph 6.5 to 8
they are also escape artists, who destroy planted plants, plastic are best, they burrow and plough the gravel/sand in your tank. indeed a recommended way of setting up a Cray tank, is to put in the gravel/sand and then throw in all the other stuff you want. over the next month the Cray will place everything where it wants it. making adjustments on a nightly basis.
be warned though they are infectious, and aren't slow to take over your life! they are easy, ish, to breed, so two can turn into ten very quickly. I love my Cray, and will defend them to the death!! its the keepers that cause most problems, not the Cray.
my advice, keep what i said in mind, and GO FOR IT. you will never regret it. if you wish to do some more research pm me for a link. only the best keep Crays imo so join them!!!
B) 
