might be OK to use a sample if it's only for a second or so;
not sure. Some bits of songs may be OK if it falls under
"Fair Use"--parody, commentary, etc.
have heard use of some song bits in other songs--Primitive
Radio Gods (I think that's the name of the band) did "Standing Outside a Broken Phone Booth With Money in My Hand" and you could clearly hear a sample of BB King's "How Blue Can You Get"
as for It's Tricky, heck, I was playing that on college
radio when it came out in late 80s...and I recognized
the sample. Long long time since it came out!
It is kind of fun to listen to early rap and try to hear where the samples come from. The Beastie Boys made a living off rapping over Zeppelin riffs.