i am new to gardening

Go ahead and plant them together, that way you will have interest and flowers in bloom for a longer period of time.

have you got any suggestions for flowers in a small backyard i have daffodils and bluebells already

are there any flowers i have to look out for that cant go together

It really depends on what you like... if you are into bulbs the crocus, snowdrops, irises and tulips will look good up until about May. Then you could have later flowering irises, lilies and sissyrinciums. There is a huge amount of choice really. Small dianthus, primulas, polyanthus, perennial poppies....... Oh lily of the valley, saxifrages, sweet peas (either the annual or the perennial), small heathers.

And no you can plant any combination of flowers you want together - I'd avoid putting to clashing colour next to each other but have seen it done intentionally for effect and it worked.