Meloxidyl Oral Liquid for Dogs and Cats

This depends on how many your Vet prescribes you based on your pet's need and dosage.

Meloxidyl Oral Suspension is indicated for the control of pain and inflammation associated with osteoarthritis in dogs.

However, it can be used for managing pain due to other reasons.

You should follow the dosing advice as per your vets instructions with regards to giving with food or not. The datasheet states the following:
Oral use. Shake well before use.
To be administered mixed with food.

Yes, Meloxidyl can be safely used alongside a joint supplement like the Vetzyme Flexible Joint with Glucosamine tablets.

Meloxidyl is very similar to Metacam, both contain the active ingredient Meloxicam. They are both prescription only products though so we can only dispense them alongside a written prescription from your vet.
It is advised to follow your vet's advice primarily. There are over the counter supplements aimed at feline bladder support such as our own brand FortiFlow.

I assume you mean Meloxidyl vs Loxicom vs Metacam.
These products are all the same in terms of actvie ingredient. They have different concentrations depending on the product chosen, e.g. cat vs dog vs horse versions etc. These differences are the same throughout the brands though, for example Loxicom for dogs will be the same concentration as Meloxidyl for dogs.

Yes Meloxoidyl (meloxicam) can be used safely alongside paracetemol. I strongly advise following your vet's advice on doseage and treatment though, you should run any medication changes past your vet.

A 100ml bottle will last you roughly 41 days at the maintenance dose for your 36kg dog.

This is a very safe and effective drug, but side effects are certainly possible. The most common side effect of overdose would be vomiting and or diarrhoea (possibly with blood). They can become extremely serious if the medication is continued despite these effects. If the dog is dehydrated when the medicine is given, kidney damage is also possible. If your dog shows vomiting or diarrhoea, you should stop the medication and call your vet. Most dogs can in fact cope with a high overdose and not notice it at all, but some will suffer.

There are a great many nutritional supplements which are claimed to benefit dogs like yours. There is a small amount of good evidence to back up the claims for some of the products. Cortaflex Dog & Cat Liquid is just one of many. There are no good comparative studies to try and evaluate different products against each other that I am aware of. Personally I would say that prescribed NSAID medications, such as Meloxidyl, are of great benefit and are well proven. It is completely safe and very often done, to also give one of the supplements concurrently. (The NSAIDs themselves are not completely safe. They will occasionally have serious side effects, but that is rare and they are of significant benefit to many animals and people.) If you are going to give a supplement, personally I would suggest Yumove Chewable Tablets. It has the best range of well-regarded ingredients and good quality ingredients in my opinion. There are plenty of rival products with similar ingredients though (about 40 listed on our website alone).