Can you make a diminutive from any word just by adding the appropriate ending such as - ín? or am I going too far?
In the story, there's a small shovel and bucket. Would that be simply sluasaid bheag agus buicéad beag or would it be possible to say and write sluasaidín (and does that change its gender?) and buicéadín ?
They had to be pretty small as she was hiding them somewhere in the top of her dress - but of course they're magic, so they do all the work that she asks them to do.

Of course it is possible.
sluasaidín, an tsluasaidín,
buicéidín, an buicéidín (slender d)

"An spadal a thugtaí ar an tsluasaidín a bhí leis an gcéachta a ghlanadh ." -- Tomás Ó Máille, An Béal Beo
"Is amharcann tú go smigshásta ansan orm Á chnuasach chugam arís Lem mhála plaisteach is lem shluasaidín" -- Seán Ó Tuama, Coiscéim na hAoise Seo
"Dara Beag bíogtha lena bhuicéidín dearg, sluasaidín sa lámh eile faoi réir le dul ag tógáil caisleán." -- Joe Steve Ó Neachtain, Clochmhóin.

Gender doesn't change (in common case) - except the meaning of diminutives is very different. In this case, they are generally masculine, e.g. cailín, paidrín
But they usually have masculine genitives. (GGBC: Ainmfhocail Díorthaithe dar Críoch -ín, 8.31)

Go raibh maith agat.