Do not use the paste sold for large livestock like horses. It's intended for much larger animals and almost impossible to dose correctly with small animals like rabbits(for most people not experienced in mixing/compounding medications), and may not be mixed evenly throughout the tube which could lead to accidental overdose. I know of instances where an OD has happened with owners rabbits. If despite this warning you still intend to use the horse dewormer paste, I would make absolutely sure it is mixed evenly throughout by squeezing out the tube into a dish, mix it up well, then dosing needs to be based on how many mg of fenbendazole are in the tube measurement(which I believe is in grams), and I would calculate off of that the exact amount needed per the body weight of the rabbit.
NEVER give an approximated pea or rice size dose. I would take the time and measure it out correctly. More than one rabbit has been overdosed and even died from toxicity due to giving a pea size amount of horse dewormer. And underdosing isn't much better as it just creates parasite resistance to the dewormer used.
I personally would use the safeguard goat dewormer liquid, 20mg/kg, and I would give once a day for 5 days straight, wait 10 days then give one more dose. The 5 days is recommended in some places as pinworms can be difficult to clear up and the 5 days is to make sure you get them all.
http://www.medirabbit.com/EN/GI_diseases/Parasitic_diseases/Pass/Pass_en.htm
http://wildpro.twycrosszoo.org/S/00dis/Parasitic/Pinworms_inf_lagomorphs.htm
Dosing is based on the weight of the rabbit. Using the 100mg/ml suspension(shaken thoroughly beforehand), it comes to about 0.09ml per lb of body weight, or I would just round that up to 0.1ml/lb and give an 8 lb rabbit 0.8ml(always check with your vet for correct dosing).
Under the fenbendazole drug page it says it's generally considered safe for pregnant animals, but I have no idea about nursing ones, so like majorv said you should consult with your vet.
Probiotics are unnecessary with dewormers as probiotics are used to provide beneficial bacteria, and dewormers are antiparasitics not antibiotics. Though if you want to give it that's up to you and likely won't do any harm.