Did Caesar reduce the number of slaves?
Possibly, but we have no reliable statistics
Caesar was concerned that there were too many slaves to be properly supervised in the countryside. His reforms may well have led to a reduction in the number of slaves but, as we don't have solid data, we can't be sure. Citing Suetonius, noted historian Richard Billows writes:
… since the great slave uprising under Spartacus it had been clear that the enormous scale of the slave economy, and harsh conditions along with lax supervision, were a serious problem, yet nothing had been done about it. Caesar now ruled that one-third of all herders in the Italian countryside must be freeborn, thereby ensuring much better supervision of the slave herdsmen, improving conditions for them since they would inevitably have similar working and living conditions to their free fellow herders, and provided work for many thousands of impoverished free Italians.
Source: Richard A. Billows, 'Julius Caesar, the Colossus of Rome' (2009).
Somewhat ironically, Caesar himself was partly responsible for the large slave population as, during the Gallic wars (58 to 50 BC), he had enslaved huge numbers of Gauls; Plutarch put the number at more than 1 million, though many of these probably did not end up in Italy and Plutarch may well have exaggerated. The whole area of slave numbers in Rome is hugely speculative for
… hardly any genuine statistics are available …
We do not know the number of slaves in any particular community of Roman Italy or in a particular sector of the economy at any given point in time, let alone for the region as a whole.
Source: Walter Scheidel, 'The Roman slave supply' (Princeton/Stanford Working Papers in Classics, May 2007).
On the issue of citizenship, the article you cited is on firmer ground but we should be careful not to exaggerate the extent to which he granted citizenship. According to Dio 41.36.3, in 49 BC, Caesar
… to the Gauls living south of the Alps and beyond the Po he gave citizenship because he had once governed them.
Also,
Caesar awarded Roman citizenship to the people of the Spanish town Gades in the same year (Liv. Per. 110; Dio 41.24.1.), but this in itself tells us little about his attitude to any large-scale extension of Roman citizenship beyond Italy. Brunt goes only so far as to say (1971: 239), “It is agreed that Caesar was much more ready than Republican statesmen had been to enfranchise provincials,” and he cites the Trans-padani, Gades, and “conceivably some other provincial towns.”
Source: Miriam Griffin (ed), 'A Companion to Julius Caesar' (2009).
Elsewhere,
In the East, no Greek cities received either citizenship or Latin rights, though several prominent Greeks and others received individual grants (Cic. Fam. 13.36, Phil. 13.33; Plut. Cic. 24). So did teachers of the liberal arts and doctors, many of whom will have been Greeks, settling in Rome (Suet. Iul. 42.1).
Source: Griffin
