If these were dogs with no issues, I would advice you simply to let the dogs sort themselves out, but both of your dogs have some degree of anxiety, and I really think it would be advisable to get a behaviourist in to assess what is really going on between your pooches.
I do, however, want to comment on a couple of points...
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Myself and the kids can take anything from him without protest or a care in the world." Please, please, please don't do this. This is a very easy way to CAUSE guarding behaviour. Your children should know never to take something from a dog... and you need to train a dog to "give" by swapping with treats and rewarding any voluntary "gives".
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Darla has been an only dog so has been a little spoilt with an endless bowl of food (she is not a greedy dog) has been allowed up on the furniture with us I do have to put her in her place now and then as she tries to nip/play with me (a harsh word or two and she stops)" - do I assume she is now on set meal times if you are needing to feed seperately?? A good idea, but do bear in mind that this is another change for her to cope with at a time when she suddenly has a lot of new things to cope with. What do you mean by "put her in her place" and a "harsh word or two"
Biting in play isn't uncommon in adolescent cockers, and simply distracting with a toy (or a scatter of treats), or halting play briefly (if biting occured during a game) or walking away is normally enough to teach them to control their teeth. Dogs do naturally communicate with their mouths a lot, so it's quite a learning curve for them not to use them on their human companions...
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Darla wont let him Loki on the sofa ( I assume she is being dominant)" Why would you assume this is dominance? She might just want some peace and quiet... or she could be "guarding" either the comfy sofa or you... it is this kind of interaction that a behaviourist really needs to advise you on, as without seeing the accompanying body language from both dogs the wrong advice could actually cause you more problems than it will cure.
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She wont leave him alone when he wants to rest she bows down wiggles back and growls to entice him to play (should i let her carry on or stop her?)" In general I would advice to let them sort it out themselves, but as your pup is undersocialised he may not have the skills to do this... if he is getting uncomfortable I would intervene by just encouraging Darla to settle for a while, but again a behaviourist will be able to see whether Loki is uncomfortable with the interaction or not.
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She often licks his ears when he is laid down (again I assume she is being dominant)" - this is actually an appeasement behaviour... and while it wouldn't worry me in the slightest in normal situations, the fact that Darla has some anxiety (manifesting as her separation issues) I would really want to observe further, as it could be a sign of deeper insecurity about Loki's arrival...
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Quite often when Loki lays down she jumps around him in circles and then cocks her leg on him without urinating (this boggles me)
When they play he chases her sometimes she puts her tail between her legs and runs away, but then comes straight back for more (this confuses me)" - Sounds like perfectly normal immature play behaviour to me.... but again, given the whole situation I would want someone professional to actually observe.
On their own, and with well socialised and issue free dogs, I wouldn't be concerned about any of the behaviours, and in all liklihood your two pups (at 1yr old Darla is still very puppy-like, although technically she is now an adolscent, but don't make the mistake of expecting her to behave like an adult dog as that isn't going to happen...) are just adjusting to each other and left to their own devices would settle into a happy companionship. However, given the whole picture I would reiterate that you would be best served getting a good behaviourist (not one who is still preaching dominance or pack theory - you want someone who uses kind and positive methods only and has kept up to date with modern behaviour theory and practice) to observe the 2 of them together in case there are subtle signs that something more complex is simmering below the surface - a bit of action now could save a lot of heartache in a few weeks' time.
