You can only ready each maneuver once at a time, but you can ready that maneuver any number of times per day; though there's little need to since your readied maneuvers refresh after each combat, and you only need a short bit of time to ready a different set of maneuvers (10 minutes IIRC).
Multiclassing into a martial adept class works fine. When you do that, your initiator level for maneuvers is equal to 1/2 your other class levels + your full level in whatever martial adept class you just took; so a cleric 6/warblade 1 is a 4th-level initiator, immediately capable of learning maneuvers and stances up to 2nd-level, though his/her number of maneuvers and stances known is based on their Warblade level, not their initiator level. If you later increased your cleric level to 8th, that would yield another +1 to initiator level (though 7th-level cleric would not, unless you also dipped 1 level into something else beforehand).
Going Warblade/Crusader or somesuch would result in two separate initiator levels; a Cleric 6/Warblade 3/Crusader 1 would count as a 6th-level initiator for using Warblade maneuvers/stances (and for determining what ones they could learn), while counting as a 5th-level initiator for using Crusader maneuvers/stances and determining what ones they could learn as a Crusader (one more level in any class, in that case, would improve both effective initiator levels by 1).
