You can use both, but you will confuse aptitude in the process.
Aptitude keeps a database so that it knows which packages it pulled in
as dependencies so it can remove them when you remove a package (so long
as no other package is using them). If you pull things in with any
other package manager, be it apt-get, synaptic, adept, gdebi or
kpackage, aptitude will not know about the dependencies that those
package managers installed and could present problems the next time you
use aptitude because it may remove things that other programs need.
To sum it up, the best advice is to use aptitude exclusively if you plan
on using it at all.