to start off with, i know nothing about radiant heating, i plan on heating the floor in my basment with a 2-ton geothermal system,  i bought all the Pex i need on sale in Menards, its 1/2 ID Dura-Pex, the only thing that represents a brand name is 'CPI'  it lists all the usual codes it meets compliance with.  it is my understanding that the manifold system for this pex is pretty simple, slide a colar on the pipe, slide the pipe over the manifold nipple, crimp the colar. my Geo installer is concerned that this method will restrict the flow to much, because the ID of the manifold nipples will be less that 1/2". he explained to me that he normaly works with wirsbo, and the pex is stretched over the manifold nipple to maintain a true 1/2 ID. my question is this, can my non wirsbo pex be used on the wirsbo manifolds?

Yes, but I don't believe durapex is a PEX-A and so wirsbo's expansion fittings won't work.

Your geo installer has no reason to fear unless you're engineering your flow rates and have a reason to be concerned. Compression and crimp inserts are used every day with no problems, it's not like Uponor/Wirsbo fittings are the only ones that work.

That said, your Durapex may not have an oxygen barrier, which may impact pump selection and such.

Sounds like your geo installer bought into the sales hype of one of the biggest OEMs in the radiant heating market hook-line & sinker! Wirsbo’s ProPex “stretch” memory fitting system does have a slightly larger inside diameter compared to crimp ring and compression fitting designs but, it’s still not a “true 1/2” ID” as your installer says. Any Mfg. will tell you that theirs is a better fitting than anything else out there and that it will only work on their tubing but we know better:-) It is important to note that the Wirsbo (now known as Uponor) ProPex™ expansion/memory fitting system can be used on PEX A only. Durapex™ (a Nibco product) is PEX C. That said; you should stick with the manifold & fitting assemblies supplied with your product line and tell your geo installer not to worry about what will amount to as an EXTREEMLY small pressure drop unless, you’re really pushing the limits of your loop lengths. Even then I still wouldn’t worry about the fitting drop. It’s a non-issue. More importantly is to make sure they sent you the orange radiant tubing or you’ll have free oxygen eating up any/all ferrous components in your system. You don’t want an open system.

my durapex is the orange variety, i underestimated how much pipe i was going to need, i will put this 1000' of durapex in my garage floor with the proper manifolds and fittings, i measured the ID of the crimp style fittings, they all have a ID of 5/16". i will play it safe for the basmement floor and buy wirsbo and use the propex fitting system. my geo installer said he has had 3 customers that used the more restrictive fittings and they had to install a booster pump to get sufficient flow, otherwise after extended run times the geo-pump would go down on high temp. he said he can sell me wirsbo for around 40c a foot, and $30 for thier new plastic manifolds. but he also told me to shop around. is this reasonable? i paid 20 cents a foot for my durapex.

I don't buy into your geo installer's story of crimp fittings being the culprit causing the need for a larger/higher head "booster pump". If you use a 3 speed pump such as the Grundfoss 15-58 all he would have to do is turn the selector switch to the next speed. In any event, it's more likely that the loop lengths on these jobs were way too long causing the thermal constipation, and NOT the fitting. Why not get some more Durapex and trade in your manifold for one with more outlets? We always use manifolds with flow meters so we can see the actual flow rate. (Not sure Nibco's manifolds have this option.) The choice is yours but there's something to be said of a distribution system with some uniformity. I find that the Uponor (Wirsbo) materials are the most expensive on the market. They are very good, but are they really worth that much more? We use Mr.PEX tubing with Upanor's ProPEX fittings. It works just the same if not better than on the HePEXplus. Mr. PEX tubing is PEX A and much easier to work with compared to everything else I've used, it's less money and my favorite color is red:-) That said, if you need to buy another manifold and more tubing; just use whatever your contractor is selling regardless of price. (His price does not seem out of line.) Sometimes this makes the job go much more smoothly. You don't want to argue with your installer over what fitting system is the best.