TinyPortal 1.6.x and 2.0.0 are coded to work with all versions of SMF 2.0* and with SMF 2.1RC2.
It is not a good practice to run older versions of SMF, PHP or TinyPortal. You should always keep your server and forum updated to the latest (supported) versions so as to keep up with any security improvements.
Notes for 2.1 installs: Note 1: due to continuous development in SMF 2.1 Tinyportal only supports the latest official (pre)release. At this time this is the RC2 version, released in March 2019. For this version of SMF you should ONLY use TP version 1.6.7. or 2.0.0
Note 2: with SMF 2.1, SMF is enforcing 'STRICT' mode in SQL, requiring certain database rules. These rules are incorporated in TinyPortal from version 1.6.1. Should you consider upgrading a forum to SMF 2.1 while using an older version of TinyPortal, it is recommended to:
update your TinyPortal version on 2.0 the the latest version available and deal with any issues in 2.0 before upgrading SMF;
uninstall TinyPortal prior to upgrading (NOT removing the data) and;
install the latest TinyPortal version on the upgraded forum: the Tinyportal tables will be upgraded in the install process and the data will be preserved.
Miscellaneous: In order to use the SMF image proxy the minimum required TinyPortal version is 1.4R.
Starting from Tinyportal version 1.5.0 the minimum required PHP version is 5.4. Installing TP on a forum running an older version of PHP is technically possible, but will result in the forum being unusable due to PHP errors.