Yes, it is ok to add bleach and cya at the same time. Put the cya into the skimmer slowly. It will dissolve slowly in the filter. May take up to a week so don't backwash or test for in in the meantime. I usually run my pump 24/7 for the first couple of days after I add cya. You can either slowly pour the bleach in the skimmer which is what I do or slowly pour it in front of a return jet.

A few hours later you can also add the muriatic acid. Pour it very slowly in front of a return jet, being very careful with it as Janet explained above.

EDIT -- Just reread my reply. Geez -- how many times did I use the word slowly??!!