Mevon
The uTasker project is not designed for any particular IDE or compiler - it works immediately with a number of such and has been tested with them but it can work in any (even if not tested and maybe needing a tweak). It also uses no libraries so all code is source code - it doesn't even use GCC startup or standard libraries and so supplies everything - even memcpy() function (which use DMA for acceleration in the K20).
I also don't understand why you would want to avoid Freescale Codewarrior. It is free up to 128k code size (special edition which is even supported by Freescale in case you have questions or difficulties) which is enough for most work - I develope industrial projects with K60s and K70s with USB and TCP/IP stacks and never reached this limit yet.... If a project goes above that limit then all you lose is the capability to debug since you can still build the project with GCC, which is what Codewarror is doing anyway.
There is a step by step of how the Kinetis starts up here (
https://community.freescale.com/thread/321229 ) that I wrote for someone. If you use VisualStudio express edition (also free) you can simply step through code and learn many details without HW and debugger restictions. Then you can cross compile what you have learned with the IDE/compiler of your choice and do the same thing on the HW or else just watch the HW work (if no debug capabilities) but in the knowledge of understanding exactly what it is doing.
The details are the same for the Teensy or any other HW that you use, from a KL02 up to a K70 processor.
Codewarrior is Eclipse based. The Freescale libraries are based on a number of projects for a number of typical IDEs and processor types, which is not always the best approach. Often the one you want is missing or the particular project that looks close hasn't been updated to solve bugs (or maybe not actualyl been tested...) and so it is hit and miss what happens and how long it takes to get it operating (correctly or still buggy). The uTasker project takes a different approach and has just a single project that automatically works on all processors - just select the family (eg. K20) and the board (eg. Teensy or add you own new configuration) and build. For example, a USB stack needs just about three lines of code that are processor dependent (mainly due to the way that the clock is derived) and not one project for each processor type and certainly not one for each IDE...
I never used the Arduino IDE. I do regularly use 9 different IDEs but at the end of the day an IDE is mainly an editor (most people have their favorites - mine is VisualStudio) and so this part has little to do with the embedded project, a make file manager and debugger (Eclipse offers lots more besides but these components are probably not used much by most people -
with exceptions as always). The make file part can be performed by hand in any editor (if needed), leaving the debugger as probably the most important feature. In fact there are many possibilities to debug today including simply adding a plug in to VisualStudio (
http://visualgdb.com/ ) [although such plug ins only work with VisualStudio full version and so are not free - if the goal is to do things without spending any money i.e. investing, then some restrictions may imply].
It depends what you want to achieve. If you want a Teensy project in Eclipse simply take Atollic Lite or CodeWarrior special edition (both Eclipse and supporting all debuggers and free), and import the uTasker project (that is, use the menu to open the project in it). Then select K20 and Teensy, click the build button and you have the solution. Then load the output to your Teensy and modify the project as desired to complete your own application.
The Teensy does support target debugging but you can already simulate and debug in the uTasker simulator (VisualStudio Express - free) and then build in the IDE to verify on the target.
If you want to debug on HW simply order a Freescale Freedom board - they cost about $14 and include an on-board debugger). Then debug any new HW level code there and use it on the Teensy later.
The USB-MSD project for the Teensy was developed without a Teensy (just in the simulator) and worked first time on the board (although it had been tested already on many other Freescale boards). I only actually received a Teensy to test with later (a friend of mine verified that it was already working).
There should basically be no problem with mixing libraries of different sources; there should be no reason to restrict to a single source of code. If you want to use some Arduino library code then why not and if you find libraries outside of Arduino that suit you better then also why not...?
So you have all that you need at your finger tips but it is up you you to start making it happen ;-)
Regards
Mark
P.S If you are worried about registering at the project, just send me your email address and I can mail you the access. The registration just helps keeping track on support cases since the project supports a number of processor types (not just Kinetis) and often people forget to inform which chip, compiler etc. they are using (in the registration these details are saved so they can be looked up without first having to ask these type of questions when responding). 
