I have 3 small (toy size) pups the 2 smallest or not getting enough milk there not weak weak they crawl around looking for mum but they dont latch onto mum for very long they have lost a few grams but i want to no can I use a puppy replacement milk (welpi) as a topup maybe just a half a 'ml after they finish feeding from mum bcz I'm being told I can then i can't bcz two diffrent milks might cause tummy problems

1.  How old are these puppies?
2.  Are they quiet after they've had a feed?

If possible, I'd avoid using bitch replacer milk unless you feel this is absolutely necessary.   You could use warmed goats milk which is close in make-up to bitch milk.   But unless they are noisy, or mum isn't producing milk for some reason, I'd leave supplementing alone, for now.  A quiet litter is a content litter.   Are they being kept warm because if they are new-born, they can't regulate their own heat and if they become morbidly chilled, they won't nurse.    Dropping a few pounds even isn't abnormal during the first few days, especially with a 'sectioned litter.

Thanks for your reply they feed but i have to help them latch on they search for the nipple but dont seem to latch on without help witch i dont mind doing but they allways come off as if they dont have a strong latch they've lost weight over past few days just 2gms at a time I set my alarm for every 2 hours to make sure they get fed but its the latching on thats hard for them I think. Mum has loads of milk I milked her to top up the 2 little ones but its really hard to milk dogs

I do have Welpi puppy replacement milk but im afraid it might give them tummy problems if i top them up with it aswell as feeding from mum bcz shes a good mum

Sorry I forgot to say there 5 days today 29/9

Have you checked to make sure they don't have a cleft palate?   It would be odd for them all to be suffering from this however.   Even if the whelping was difficult, by day 5 they should be forging ahead.   Again are they being kept warm (external heating to arouind 75-80F) and are they quiet after feeding?   If not, either, you may need to involve your vet.   It sounds as if they are a bit weak.   We have kept newborn puppies going with a glucose solution, initially.   Do you have a mentor (sire's owner?) you can call on to see what's going on?

Did you vaccinate against CHV? I think I would be seeking vet advice in your position.

I would try firstly giving them private nursing sessions on mum.  That involves removing any stronger puppies to a warm separate box with a heat pad or lamp on it, and letting the weaker ones have time alone with mum to nurse.

If they are still not nursing when given exclusive access to mum, then it's likely they are too weak to nurse for long enough, which is a downward spiral that leads in a direction you don't want to go in. 

In which case, you can DEFINITELY supplement with a replacement milk and NO, it won't affect their stomachs to have two different types of milk in there.  I would let them drink what they can from mum and then try supplementing - but if they can't nurse from mum, you may find they are also too weak to suck from a bottle, as that also involves sucking and strength. 

In which case, you will need to tube feed.  If you go to YouTube and search for tube feeding puppies or 'Myra Savant Harris' and tube feeding, you will see some videos showing how to do this.  It is scary at first but it's actually incredibly simple and fast.

PS You should post any future questions in the Breeding forum of this group, to get the help you need.