Can we evict tenants in a 2nd floor apartment because they are too heavy and the ceilings are cracking?

If there is no lease, then rather than pinning the eviction on weight--which could implicate non-discrmination laws (even if the tenants lose, there's a good chance that they'd at least raise the claim, costing you time and money), simply give them an eviction notice at the end of the month; rentals without leases are typically construed as month to month. At the end of each term (in this case a month) of a lease, landlords may refuse to renew and may therefore evict if the tenants don't leave.

Going forward, rather than write weight into a lease (and you should have a written lease to protect yourself; note that a written lease can still provide for a month to month term), put in  a clause that tenants coventant to not damage the premise, including common areas or areas occupied by other people, and that tenants may be evicted if they cause such damage. That will cover you more broadly, without making a specific physical characteristic a linchpin.

In terms of recruiting tenants, if you are a small unit (e.g. renting out the upstairs of a home or the equivalent) you have substantial discretion in who you rent to--simply don't rent to people you suspect will damage your premises.