u can definitely commune in a coptic church as an armenian orthodox Christian, but usually it is good to meet or telephone the priest beforehand, or stand in the queue (line) with someone the priest recognises, mainly so he knows you go to confession regularly.
this is to prevent you receiving judgement from God for communing without repentance.
only once i communed before knowing the priest, then i was late as well!
i went up to the priest and asked 'i'm i bit late, abouna (father), may i take Holy Communion?',
to which he nodded and gave me Holy Communion.
he came up to me at the end of the service to greet me and apologised for changing the service time to
start earlier than on the website! (one of the humblest priests i ever met) i then explained where i was from and in which church i became orthodox (being a very pale person, i was obviously not egyptian). i learnt very much from this priest and others in the church.
but generally you should speak to the priest first.
major coptic liturgy 'rules' (also common in some other churches):
always stand up for the gospel.
also stand up whenever the deacon says 'stand in the fear of God'.
bow your head, do a waist bow or kneel or prostrate when the deacon says 'bow your head before the Lord'
and also during the absolution (starts with the words 'your servants, oh Lord') towards the beginning of the service. this is a good time for remembering your sins and repenting.
other than this, you can stand, sit or kneel as you think is appropriate.
contrary to popular practice, copts are not supposed to kneel on sundays, but most don't realise it.
i think it's because church is often on a friday in egypt.
i was immediately recognised as a copt by an antiochian priest once because i had kneeled during the great entrance (when the Body and Blood come around the church). i didn't know about the sunday thing then.
never kneel after Holy Communion! you have taken the Body and Blood of Jesus, so you should not make any prostrations till the next day.
we don't take Holy Communion during our monthly periods (send p.m. for any questions or thread will be derailed!), nor does a man after a nocturnal emission of semen or anyone with a bleeding cut.
yes, take your shoes off for communion. we can't believe people commune with shoes on, it's just a cultural hang up, not an article of faith, but it's a strong cultural hang up! for us it would be like taking your shoes off in the house and putting them on an expensive sofa or on the dinner table.
having said that i forgot once!
(it was a very cold day)
i noticed on the way back, and just managed not to freak out publicly!
so, also, bring really thick socks if your area has cold winters! put them on over your normal socks as you go forward. i have communed in 3 pairs of socks before when it has been below zero with broken church heating!
about when to make the sign of the cross, lots of people do it at the end of the service when apparently they shouldn't. i don't think it's a very big deal.
we generally make the sign of the cross a lot. it should be at times when it fits in with your personal prayers (echoing the public prayers around you). so it's not such a synchronised thing. you don't have to enter a competition to see who does it the most, it just looks like that!
oh, and other 'rules':
if there is no projector, pick up a service book so you don't get lost.
anyone with english writing on it will do (unless u speak arabic). it will be called the 'kholagy'.
if / when you get lost, don't worry, if you can't understand what is going on, read parts of the kholagy and pray to God.
if they say abfjafd abadfjb;aej bajb;iaer (2 minute mumble), that's the Lord's prayer (said fast with a thick accent in english, it is not clearer than in arabic!) and e;kf aiog djaf ephr adjf uhbhb aekrj cyda njt (5 minute mumble) is the nicene creed.
on one of those times you get lost, look out for these parts in the service book in advance and mark them (unless you have memorised them) so you can join in during the mumble.
the Lord's prayer is said at least 5 times, so if you recognise it, it's a good way of working out where you are in the service.
don't worry to much about the rules. i have broken all of them and didn't get excommunicated (yet).
remember to pray to God and he will guide you through!
edit: could vas tearn give us a similar list of what to do / not do in the armenian church?
then i can go and visit :-)