Hi basel
You can work with an L permit as these are issued i beleive for short term contracts that have a fixed start and end date. You will have to pay each time it is renewed (i could be wrong).
I have had an L for 18 months and the first 9 months i had no job and the L permit stated i was searching for work. Now i'm working (contracting) the L permit now states i am employed but i still have to renew on expiry and pay 65 CHUFS....
With regards to agencies there are several besides the ones you mentioned but also some english ones also have roles in Switzerland.
ELAN IT - offices in Basel, Zurich and Bern
Humanis
A-team
Swisslinx
Darwin recruitment
Swisslinx
Swissselect
Nexus
ITcag
These are to name a few but not all deal with Basel....
the following websites may help for your particular field.
www.jobsuchmaschine.ch/ http://job.speed.ch/
Then there are the regular job sites such as monster and jobserve. If you definitely want to work in Basel then i would start to list all the companies that use English as a language and AS400/Admin and target them.
of course there are other people who use this forum who may be able to help as well.....
Good luck
Matt