It is not clear that, whether hydrogel particles are going to compress into tablets or coating drug granules with hydrogel forming polymer matrix. Based on this it is possible to decide whether filler/diluents required or not on trial n error basis. Wish you all the success. Please don't forget to upvote my answer.

Regards

You can use filler based on your pre-compression and release. If your release of the drug  and precompression parameter are complies as per specification then there is no need of filler.Diluents provide the required bulk of the tablet when the drug dosage itself is inadequate to produce tablets of adequate weight and size.Maybe this will be helpful. 

According to my experience the minimum weight of tablet which can be easily handle and compressed is 80-90 mg. Average weigh of tablet less than this is difficult to handle and also required sensitive tooling. The tablet which contain very potent drugs e.g. hormones, required very small dose. For these tablets we require diluents. The dose of your drug is 150 mg so you can choose any material for hydrogel matrix. The matrix materials along with other excipients will increase the weight of tablet sufficient enough so you will not require diluents for same.

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Yes but if your tablet is already greater than 300mg and you have to use a larger capacity machine. Otherwise, you can use or prepare small tablets as well.

Regards

You may use some fillers, if depends about the matrix compression mechanism (plasic or britle deformation) and the API solubility properties,

fillers or diluent not required. try to make tablet as small as possible provided that you get your hydrogel formulation advantages.