Upgrading disk in late 2015 27" iMac

1. no, you can use a different size HDD and/or SSD than stock.
2. i would recommend shopping at OWC for the replacement drive(s)
3. i think it would be hard (if not impossible) to find an SSD larger than 4TB for the SATA drive and 2TB for the NVMe drive.
4. if you only want one drive, you should replace the NVMe drive with the largest one you can afford and either remove the SATA drive completely or replace it with a larger HDD. it would be much faster than using the SATA drive as your primary drive. i personally would use the HDD as "external" storage instead of trying to recreate the fusion drive. but that would just be my personal preference.

and please find and watch the install videos on OWC’s website for your specific model to see if you can replace the hardware yourself. it's a very involved operation on a 2015 iMac and it might be better to have the hardware replaced by a professional.

I have a late 2015 27" iMac

That model has limited drive upgrade options per one of the biggest suppliers of SSD upgrade kits:

See the Compatibility listings on this page: OWC 2015 iMac SSD upgrades

I've been having weird drive-related problems

If you post details of the problems we may be able to save you some money. The local service provider charges US$80/hour and requires two hours to do any work inside sealed-case iMacs (2012 and newer). That's $160 on top of the cost of the drives and the resealing kit. That could really add up if you install a 1TB or larger SSD.

One things to check now before throwing down money is covered in this Apple support article. YOu cah do this at home:

How to fix a split Fusion Drive - Apple Support

That is a major cause of Fusion drive issues.

I don't believe iMacs made after 2011 require the drive temp sensor. The above company does not show a temp sensor as a part of their drive install kits for newer iMacs like it does for 2010 and 2011 iMacs.

Thank you. With that set of issues I believe we can all save typing a pile of followup questions if you post a configuration snapshot. Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

No personal or security is revealed. Please this excellent user tip on posting text reports like EtreCheck's.

How to use the Add Text Feature When Post… - Apple Community

1. Any capacity will work.
2. VERY unlikely.
3. None
4. Yes.

Notes: Do not do this yourself, halve an Apple Authorized Service Provider do the work. Insist they install a fan sensor cable.