Alien Dj u dont have to mix the tracks from head to tail, u can pick up the right 'block' to mix it in with the other comin track... bit of course they are rules in a song structure, they r bars n blocks, as long as u respect those n dont cut them off, u can end up ur track when ever u want.
now about the tonalities, u can hardly mix a trance track with another in the part where the melody or keys or strings r playin without kicks or beats....also this might kill the track, cus the strings nd keys and melodys r the heard of a trance track.
beatmatching is MAJOR of course.
You can mix all kind of trance and its sub genres but u have to do that smoothly and in harmony, u canot play a vocal trance with a hardcore one then a commercial followed by a prog house specialy if ur doing a mix CD.... here comes the skills of chosing the right playlist, you should chose ur tracks and depending on ur knowledge n skills u setup the right schedule of mix.
Now if ur doin a mix CD its better and always more appreciated if u play as much as u can from the tracks, usualy some DJ's play short periodes of the songs when they r doin a commercial CD, or when they r spinin at small gigs, they do that to not let the crowd down n bored ( thats usualy when the crowd is not quality).
as an advice, keep ur mix pro and let the whole track play, or as much as u can but dont make it short, let the beauty and the MESSAGE of the track be heard and understood.
Always ALWAYS select ur playlist and be accurate b4 doin ur mix session, do not mix on the spot using random tracks.
for me even if i go to a gathering party when i am invited, if they ask for me to spin just to show skills or even to playmy own style, i say okey but depending on the time i have, i get my bags of cd's, and select the tracks i wanna play, and then i do my mix.
To always be in a high pro image.