when you say about building my own loops using a variety of instrument sound sources, do you mean assigning each of these to individual notes on the keyboard, or combining these rhythmically to create a short (or long) musical phrase ??
Both!
Assign the samples across the keys, then jam with them. You will find the groove will have its own sonic character.
Remember to map each sample over enough keys to alter the root note and perform riffs.
Record the parts into a sequencer and try out variations of the groove, song progressions and arrangements.
Don't get to precious about a sample, if it ain't working try out a new one.
If the one shot samples timing sounds way out as you move away from the root note , you could create copies of it and pitch change them as mosrob points out., this could be used to build an instrument sample that doesn't become shorter or longer as you move around its root.
Keep your ears open for exposed stabs chops and hits that appear without other instrument layers behind them.
These usually make the most usable samples.
If you are worried about breach of copyright then use only recordings that are not effected by these legal issues.
Eg. Music more than 50 years old (pre 1963) plenty to choose from here - blues, swing, rock n roll, jazz, classical, Latin.
Track using latin and swing samples, link..
http://youtu.be/jvQgBrQnqzkor you can always use library music such as that on the KPM label
Some great sounds can be had from these tunes, even if the music sounds a little cheesy.
Example in Link..
http://youtu.be/G-NR5MvETpYI know you're a working musician and this can lead to opportunities to get yet more samples.
It's worth considering recording jam sessions with musicians you know. If you split the signal from a guitarists audio channel and capture the whole performance onto a iPod without the other instruments mixed you will have loads of material to chop and process for future use.