depends on a lot of things

Very short wheelbases can loose power at swtiches, unless you have room for some form of keep-alive.  Make sure that, whatever loco you get, it has power pickup on both trucks.   i was impressed by the Walthers Plymouth ML-8; it has a capacitor.  (I saw it at a train show; I do not own one, so can't give feedback on how well it runs over time1)

Diagnosis

Dear ??? (kh25),

Sounds awfully like you were seeing issues, but may not have fully diagnosed the causes.

Firstly, was your SW7 a Walthers-era "P2k switcher" or a Life-Like-era "P2k switcher"?

As I recently learned here onlist,
(always learning, always learning...   ),

while both are commonly and interchangably referred-to as "Proto 2000",
the original LL "Proto 2000" versions had proper all-wheel-pickup,
whereas the newer "DCC + Sound equipped" Walthers "Proto 2000" versions feature(?) at least one axle with rubber traction-tyres, which immediately cuts down on the effective "number of points of pickup".

(See the following thread)
http://model-railroad-hobbyist.com/node/32421

Having used 4-axle (EMD SW-type and Alco S-type) switchers from LL-P2k, Athearn, B'mann, and Atlas on PECO Insulfrog (plastic frog) turnouts from the Setrack, Streamline, and now "US Geometry Code 83" line for over 2 decades, I can assure you that:
- The curve radii thru the turnout is likely not "the key cause of failure"
- Such locos can easily crawl thru such "insulfrog" turnouts if all wheels are properly picking-up power as the design intends

Literally anything which isolates or unduly affects the ability of any given axle/wheel to make contact with the rail is a bad thing and should be rectified with extreme prejudice. As noted on a many-years-old thread here onlist

"...each wheel<> rail contact point can be considered a "vote" for pickup,
to maintain operation a loco needs at least one "vote"/active-contact per rail working at any given moment,
the inherrently intermittent-contact behaviour of a rolling wheel means that the only way to achieve the required "always at-least-one-active-contact-per-rail" is to maximise the number of wheels/"votes" making contact..."

SO, in answer to your question

The companies listed above all make gorgeous little switcher locos which should work reliably with your PECO turnouts, assuming all wheels are correctly wired/configured/tuned for all-wheel-pickup operation.
(Said in-inverse, any loco from any manufacturer can "appear to have problems" under the conditions mentioned if
- the track is dirty,
- the wheels aren't clean,
- the pickups aren't properly operational,
- and/or the loco inexplicably isn't wired for "all wheel pickup")

I hope this helps...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS DCC is more finicky about "clean wheel/rail pickup" than analog,
making the "each pickup-wheel is only a 'vote' for pickup at any given moment" analogy even more appropriate...

I feel that what is needed is

I feel that what is needed is something with more weight.  I had a few of the P2K switchers.  They run OK, but are somewhat light.  The Atlas S2 defintely has more weight, and should work better.  Also try a Walthers SW1, from the newer DCC-ready runs.  Also a fairly heavy unit. 

3 things!

Any short wheel base switcher will suffer from weight, stalling ect. 

Ireguardless of the brand,

1. Make sure it will run smooth as silk on straight track without stalling. DCC is not forgiving in power glitches especially with sound.  Do what it takes to ensure good operation.
2. Add weight where ever you can.  Even a little bit will help.
3. This is your choice: power the frogs if you can or add a "keep alive" circuit or do both!  

Personally, I have been adding "keep alive" kits to all my locomotives.  A fellow modeler has them available in kit form.  You decide where to mount the components and this allows you to fit them in really compact locomotives.  This has eliminated all my locomotives from stalling.