Can you use the sidekick rules from Tasha's CoE to make a summoned familiar as a sidekick?

Question

Can you use the sidekick rules from Tasha's Cauldron of Everything to make a summoned familiar as a sidekick?

Answer 1

Not by the rules

There already is this question on how to improve your familiar by you, where using the sidekick rules was recommended (and downvoted). Sidekicks are different in nature from summoned familiars. Tasha's describes them as special NPCs:

To join the adventurers, the sidekick must be the friend of at least one of them. This friendship might be connected to a character's backstory or to events that have transpired in play. For example, a sidekick could be a childhood friend or pet, or it might be a creature the adventurers saved. As DM, you determine whether there is sufficient trust established for the creature to join the group.

Your familiar is not a creature that trusts and wants to join the group. It is a spirit that you summon via one of your class features, it has no choice in the matter. It lacks all the abilities of a sidekick to gain experience and grow along with you (excepting a few Warlock class features that are the Warlock's doing, not the familiar's.) For example, it cannot attack, while a sidekick can.

There are some familiars you can befriend. They all are monster manual monsters, and their CRs are all above the upper limit of 1/2 given for sidekick creatures, so they do not qualify as companions. They also work differently from normal familiars, you need to encounter them during adventuring, and your DM needs to agree to allow them as familiars.

Combining companion levels with your summoned familiar in a homebrew setup could lead to balance issues, as your companion can attack, you can dismiss, recall and teleport your familiar freely, and it can not die -- you can always call it back with another casting of the spell at full strength. There is a reson the power of familiars is limited in the way it is.

My recommendation is: if you want a sidekick as a friend that tags along and gets better over time, work with your DM to get a sidekick. You can treat and roleplay it like a familiar, without needing to take advantage of the features of the find familar spell. Think Catweazle.

Answer 2

The DM decides how the Sidekick rules work and when they can be used.

The Sidekick rules appear in the "Dungeon Master's Tools" section of Tasha's Cauldron. The intro to this section reads:

The Dungeon Master employs many tools when preparing and running a D&D campaign. As a DM, your tools include your imagination, your ability to discern what entertains your players, your storytelling acumen, your sense of humor, your ability to listen well, your facility with the game’s rules, and more. This chapter adds to your toolbox with guidance and optional rules for a variety of situations. The chapter also includes a selection of ready-to-use puzzles, which you can drop into any campaign.

The tools herein build on the material in the Dungeon Master’s Guide and the Monster Manual. You may use some, all, or none of these tools, and feel free to customize how they work. Your group’s enjoyment is paramount, so make these rules your own, aiming to match your group’s tastes.

So the Sidekick rules are a tool for the DM to use at their discretion. So "can you make X a sidekick?" isn't a question that anyone can answer, except for the DM at a particular table, and they can only answer for their table.

Answer 3

Probably not, at least until WotC adds some familiar stat block in one of their books.

TCoE says:

"A sidekick can be any kind of creature with a stat block in the Monster Manual or another D&D book, but the challenge rating must be 1/2 or lower."

The familiar "has the statistics of the chosen form..." but his stat block will be different, not just by ceasing to be a beast but also reducing his list of actions and gaining new features.

Maybe you could use the variant rule "Pseudodragon Familiar" (MM, page 254) who let you get the similar link but with a creature who has the stat block.